#!/usr/bin/env perl
use warnings;
use strict;

while (my $myvar=<DATA>) {
	chomp($myvar);
	if ($myvar=~/^([A-Z])\1*$/) {
		print "'$myvar' matches\n";
	}
	else {
		print "'$myvar' doesn't match\n";
	}
}

__DATA__
X
XXXX
XXXY
XXYX
YXXX
AAAA
BBBB
BBBC
DDD4

##</code><code>##

'X' matches
'XXXX' matches
'XXXY' doesn't match
'XXYX' doesn't match
'YXXX' doesn't match
'AAAA' matches
'BBBB' matches
'BBBC' doesn't match
'DDD4' doesn't match